style(related): two-up on desktop, one-up on phones

Six photo cards in 2 columns (3 rows) at desktop widths; phones collapse to a
single full-width column and still show only the first four. Drops the 900px
two-column step (now the base) and the cramped-card type overrides that only
existed for the 2-up phone grid.

docs/design-guide.md

@@ -22,7 +22,7 @@ Read this **before** touching theme, layout, typography, color, or SEO markup. P
 - **Sticky header** — name/logo + nav (Blog / Categories / About / RSS).
 - **Article header** — title, pub date, reading time, category + tags, author card (photo, 1–2 line bio, LinkedIn + GitHub + git.hoelee.com links).
 - **Author box / ProfilePage schema** on every post (see SEO below).
-- **Related posts** (6 photo cards: 3 across on desktop, 2 across ≤900px, the first 4 only on ≤600px) — internal linking + keeps readers on-site. Each card shows the post's own language-specific share card, so an EN page never shows ZH art.
+- **Related posts** (6 photo cards: 2 across on desktop, 1 across on phones with only the first 4 shown) — internal linking + keeps readers on-site. Each card shows the post's own language-specific share card, so an EN page never shows ZH art.
 - **RSS link** in footer — how the technical audience follows me.
 - **Custom og:image per post** — 1200×630, never the reused logo.